In many microwave devices, such as power amplifiers, signal generators and oscillators, it is desirable to be able to control the amplitude of the output either automatically or manually. In this manner, the radio-frequency source output can be level controlled or pulsed as necessary.
In order to modulate the output of a radio frequency source, a modulator or attenuator is usually connected in series with the source. Generally, it is desirable that this modulator be linear over a predetermined range of frequencies and that the amount of attenuation produced by the modulator is proportional to some input signal or voltage. Such a linear modulator can be used in a feedback loop which can automatically control the modulator to hold the output of the radio frequency source constant.
A conventional radio frequency (RF) modulator design that is commonly-used in radio frequency circuits is a reflective shunt diode modulator. Such a modulator consists of a transmission line which is connected to ground at intervals by PIN diodes. In accordance with well-known RF practice, in order to reduce the device insertion loss as much as possible, the diodes are spaced equally along the transmission line and the length of the transmission line sections between diode pairs is selected to 1/4 of the wavelength of the RF signal at the high end of the operating frequency range.
With such a configuration, the device has essentially two operating states. In the first of the two states, the diodes are "off" and the device has the frequency characteristics of a filter since the "off" diodes appear as capacitances. Consequently, RF power flowing along the transmission line is not significantly attenuated.
In the second or "on" state, the diodes are biased by a DC current into a conducting state, essentially causing low-resistance short circuits to appear along the transmission line. The diode induced shorts divide the line into quarter wavelength sections at the high band end. Consequently, at the high band end, reactive cancellation occurs in the quarter wavelength sections, in turn causing the device to highly attenuate RF power flowing through the device.
The problem with such a prior art modulator is that it cannot effectively cover a typical operating frequency range. In particular, as the RF frequency drops, the RF wavelength increases and the transmission line sections connected between the diodes are no longer quarter wavelength sections. For example, at the low band end the RF wavelength may increase to such an extent that the line sections are 1/8 wavelength or less. At this point the reactive canalling effect produced by the transmission line sections all but disappears and the transmission lines appear as simple short circuits. Consequently, the attenuation of the device is simply the "on" resistance of the diodes connected in parallel.
To maintain an attenuation equivalent to that obtained at higher freqencies when reactive cancellation is effective, it is necessary to reduce the "on" resistance of the diodes at lower operating frequencies. A typical prior art method of reducing diode forward resistance is to increase the forward bias current on the diodes at the low band end. Thus, it is possible to maintain reasonably constant attenuation over a broader operating frequency range by varying the diode bias current.
This necessary change in bias current with respect to operating frequency can make the prior art modulator difficult to use in certain circuits. For example, the prior art modulator device may be used in a feedback loop in which the modulator bias current is controlled by an automatic feedback circuit which samples the output of the device and utilizes negative feedback to maintain a constant output. In such a circuit the change in bias current, necessary to maintain a constant output at changing frequency, is equivalent to a feedback loop gain change which may cause the loop to become unstable as the frequency changes across the operating frequency range.
Furthermore, the increased bias current necessary to maintain high attenuation at the low band end drives the diodes into saturation and, thus, the diode switching time is increased. Consequently, in order to use the prior art modulator as a pulse modulator with fast switching times, the operating frequency range must be limited to two frequency octaves. Consequently, the prior art devices tend to have either limited bandwidths or poor pulse switching characteristics.
Accordingly, it is an object of the present invention to provide a radio frequency modulator which has an extended operating frequency range.
It is a further object of the present invention to provide a radio frequency modulator which has a high pulse switching rate.
It is another object of the present invention to provide a radio-frequency modulator in which the bias current required to achieve a predetermined attenuation is constant across the entire operating frequency range.
It is still a further object of the present invention to provide a radio frequency reflective shunt-diode modulator which needs less current to operate at maximum attenuation and in which the diodes are operating in their small-signal region.
It is yet another object of the present invention to provide a radio frequency modulator in which maximum attenuation can be obtained without driving the operating diodes into saturation.
It is a further object of the present invention to provide a radio-frequency modulator in which pulse rise and fall times are improved over prior art circuitry.